1. Field of the Invention
The present invention relates to a spread illuminating apparatus for use with a liquid crystal display (LCD) device, and particularly to a spread illuminating apparatus employing one or more spot-like lamps as light sources.
2. Description of the Related Art
A spread illuminating apparatus of side light type, which has a light source disposed so as to face one end surface of a light conductive plate, is heavily used as a subsidiary illuminating apparatus for an LCD device. A side light type spread illuminating apparatus has a bar-like lamp as a light source, for example, a fluorescent lamp having a length substantially equal to the width of a light conductive plate, whereby a uniform illumination can be provided across the light conductive plate.
In a device, such as a portable telephone, which demands low power consumption, a light emitting diode (LED) is employed as a light source. The LED is a spot-like lamp and raises a problem with regard to achieving a uniform brightness spread across the light conductive plate.
A spread illuminating apparatus of side light type introduced to address the problem is disclosed, for example, in Japanese Patent Application Laid-Open No. H10-293202. In the spread illuminating apparatus disclosed therein, a light conductive plate has, on one end surface (light entrance surface) thereof facing an LED, a prism array structure which is adapted to laterally diffuse light emitted from the LED, and a light scattering pattern for a uniform spreading is formed on a major surface of the light conductive plate. And, a light diffuser plate may optionally be disposed between the light conductive plate and an LCD as required for achieving a further uniform illumination.
Another side light type spread illuminating apparatus to address the same problem is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-260427. In the spread illuminating apparatus, an optical element comprising an array of prisms is disposed between a spot-like lamp and a light conductive plate so that light emitted from the spot-like lamp is refracted differently depending on its incidence angle with respect to the prisms thereby changing the traveling direction and the intensity distribution of the light when entering the light conductive plate.
Still another side light type spread illuminating apparatus is disclosed in Japanese Patent Application Laid-Open No. H10-199316 and is shown in FIG. 12 attached herein. In the spread illuminating apparatus shown in FIG. 12, an LED 1 as a spot-like light source is disposed at an end surface 2C of a light conductive plate 2, and light emitted from the LED 1 enters the light conductive plate 2 and exits from one of two major surface of the light conductive plate 2. A plurality of grooves 2D extending orthogonally to the major surfaces are formed at a central portion of the end surface 2C of the light conductive plate 2 so as to oppose the LED 1, whereby light emitted from the LED 1 is diffused upon entering the light conductive plate 2.
The above-described spread illuminating apparatuses using a spot-like lamp, such as an LED, as a light source, have the following problems.
An optical path conversing means, which comprises, for example, arrayed prisms, or grooves as shown in FIG. 12, must be disposed or formed at a light entrance surface of a light conductive plate so as to laterally spread light emitted from the spot-like lamp. Such prisms or grooves must be arrayed at very minute intervals, which requires micro-fabrication resulting in an increased cost.
Some of lights emitted from an LED are not duly introduced into a light conductive plate, and are just wasted. Specifically, referring to FIG. 10A, out of lights emitted from an LED 1, lights PR do not fall incident on an end surface 2C (light entrance surface) of a light conductive plate 2 due to an air layer existing between the LED 1 and the light conductive plate 2 thereby failing to be introduced into a light conductive plate 2 while lights P are duly introduced into the light conductive plate 2. And, some of lights falling incident on the end surface 2C may be reflected thereat failing to be introduced into the light conductive plate 2. Such problems occur even when a guide rod is provided or a bar-like lamp like a fluorescent lamp is used in place of the spot-like lamp.
In order to deal with the above-described reflection loss (coupling loss) problem incurred due to the existence of the air layer, adhesive 3 may be applied between the LED 1 and the light conductive plate 2 as shown in FIG. 10B. This may ease the problem but does not cause lights PR to sufficiently change their traveling directions resulting in insignificant improvement. Thus, light emitted from the LED 1 is still not efficiently utilized, which constitutes an obstacle to lowering power consumption and increasing illumination brightness.
The adhesive 3 may be replaced by a well-known SELFOC (trademark) lens as a light converging element, which is formed of glass, shaped in a column, and which has a refractive index profile formed in the radial direction thereby functioning like an ordinary lens. The columnar SELFOC lens formed of glass as its base material is commonly used for optical communication, optical measurement, optical information processing, or the like but is not suitable for tightly covering up the light entrance surface 2c of the light conductive plate 2, which usually has a plane surface.
The adhesive 3 may also be replaced by another well-known light converging element called “cylindrical lens”, which is formed of a glass base material, is shaped in a cylindrical configuration, and which has a uniform refractive index profile. Light falling incident on the circumferential surface of the cylindrical lens can be converged only in a particular direction, for example, vertically. However, the cylindrical lens has a uniform refractive index profile as described above, and therefore does not provide a satisfactory convergence characteristic. Further, since the cylindrical lens has an arced incidence surface for converging diffused light in a parallel manner, an air layer still remains toward the LED 1 thus raising the aforementioned reflection loss problem. In order to eliminate the reflection loss due to the air layer, a transparent adhesive or resin may be filled between the LED 1 and the cylindrical lens. However, since the refractive index of the cylindrical lens is defined as relative to the refractive index of the filled adhesive or resin, the converging performance of the cylindrical lens is significantly deteriorated. A cylindrical lens formed of glass with a high refractive index and with a thickness of 1 to 2 mm may duly work, but to produce such a cylindrical lens is difficult or prohibitive in view of physical strength, manufacturing technology and production cost. On the other hand, a cylindrical lens formed of synthetic resin does not achieve required performance because the synthetic resin has a lower refractive index than the glass therefore lowering its convergence factor.
In order to be free from the cost-increasing micro-fabrication which is required to form the grooves 2D (refer to FIG. 12) for an optical converging means, concave lenses 40 may be disposed between LED's 1 and a light conductive plate 2 as shown in FIG. 13 to thereby diffuse light emitted from the LED's 1. The concave lens 40 is adapted to diffuse light with respect to all the directions from its central axis, and therefore is not suitable for diffusing light only with respect to a particular direction, for example, horizontally.
Also, a light conductive plate 2 is structured so as to have a major surface area as shown in FIG. 11, which is larger than a display screen area of an LCD device (not shown) disposed over the light conductive plate 2. Lights emitted from LED's 1 are not uniformly spread at an area 2A positioned toward an end surface (light entrance surface) 2C of the light conductive plate 2. The area 2A, which usually has a length LD measuring 2 to 4 mm in a small equipment like a portable telephone, is called “dead area”, and covered up to prevent light from exiting out, and only an area 2B of the light conductive plate 2 is utilized to illuminate the display screen of the LCD device (not shown). Thus, downsizing of the equipment is hindered.
In the spread illuminating apparatuses structured as shown in FIGS. 12 and 13, an air layer is inevitably formed at a space between the LED 1 and the light conductive plate 2, and light emitted from the LED 1 is partly caused to be reflected due to the existence of the air layer and is just wasted. To compensate for decreased brightness due to the reflection loss incurred, power consumption must be increased, which hampers reduction of power consumption.
Thus, the conventional approaches to achievement of uniform illumination across a display screen by using spot-like lamps, for example LED's, as light sources have encountered confliction of “uniformity versus brightness”.
In the meanwhile, study and innovation have been made on the directivity of light emitted from an LED, and various LCD's have been newly developed and come out with a variety of light diffusion characteristics. In accordance with the study, innovation and development, demands on light exiting from a light conductive plate are increasingly diversified. Under the circumstances, it is desired in terms of cost that a light conductive plate be provided with diversified characteristics by means of minimum kinds of light controlling means to thereby achieve a spread illuminating apparatus adaptable to LED's and LCD's having a variety of characteristics.